The present invention relates to a process for producing rare earth metals including rare earth-containing alloys which can be used for rare earth-containing alloy magnets, hydrogen storage alloys for anodes of nickel-hydrogen rechargeable batteries, and the like.
Rare earth metals are used in a wide variety of usage such as lighter flints or steel refining additives. It is known that such rare earth metals can be produced by a molten salt electrolysis of rare earth chlorides. Recently, rare earth element-transition metal alloys have been developed for permanent magnets providing high performance, and samarium-cobalt magnets, neodymium-iron-boron magnets and the like have been put into practice. Alternatively, hydrogen storage alloys providing high performance such as a lanthanum-nickel alloy and a misch metal (mixed rare earth metals)-nickel alloy have been put to use in a large amount as anode materials for nickel-hydrogen rechargeable batteries. Rare earth metals used in these alloys are required to have high quality, but the rare earth metals produced by the molten salt electrolysis of rare earth chlorides contain a quantity of impurities such as chlorine, oxygen, or the like, so that performance of such rare earth metals cannot be improved sufficiently.
In order to overcome this problem, electrolysis in a fluoride molten salt bath with a charge of oxides have been developed (E. S. Shedd, J. D. Marchant, M. M. Wong: U.S. Bureau of Mines RI 7398 P.3 (1970)), and performed in an industrial scale (Electrochemistry Handbook, Fourth Edition, Edited by The Electrochemical Society of Japan, Published by Maruzen Co., Ltd., P.399 (1985)) for producing a large amount of misch metals. According to this method, a mixed salt consisting of 50 to 75% by weight of rare earth fluorides, 15 to 30% by weight of lithium fluoride, and 10 to 20% by weight of barium fluoride, is charged into an electrolytic cell made of a refractory material, and heated to 850 to 1000.degree. C. for melting the mixed salt. Then, while bastnasite ore previously calcined and refined or refined rare earth oxides are charged into the heated and molten mixed salt, electrolysis is performed at a voltage of 6 to 12 V, an anodic current density of 0.5 to 1 A/cm.sup.2, and a cathodic current density of 1 to 10 A/cm.sup.2, using a graphite anode and a molybdenum cathode, to thereby electrodeposit and recover misch metal. In this electrolytic reaction, the oxides dissolved in the fluoride molten salt is electrolyzed in accordance with a reaction formula 2 Mm.sub.2 O.sub.3 .fwdarw.4 Mm+3O.sub.2 to form misch metal (Mm). The oxygen in the oxides reacts with graphite in the anode in accordance with a reaction formula 3O.sub.2 +3C (anode).fwdarw.3CO.sub.2 .uparw. to become carbonic acid gas, and exits the reaction system.
Alternatively, if the electrolysis in a fluoride molten salt bath with a charge of oxides is applied for producing a neodymium metal used as a neodymium-iron-boron magnet material or the like, neodymium will precipitate in a solid state at the electrolytic temperature of the misch metal since the melting point of the neodymium metal is as high as 1050.degree. C., and recovery of neodymium metal will be difficult. Thus, the electrolysis should be performed at an elevated temperature. The electrolytic reaction proceeds in accordance with a reaction formula 2Nd.sub.2 O.sub.3 .fwdarw.4Nd+3O.sub.3, and oxygen in the oxides reacts with graphite in the anode as in the case using the misch metal, to become carbonic acid gas, and exits the system. The neodymium metal can be produced in an electrolytic cell equipped with a consumable cathode. In particular, if the neodymium metal is to be produced in the form of an alloy of neodymium and iron, using an iron cathode as the consumable cathode, under the conditions to set the iron content of the alloy to be 10 to 20% by weight, the melting point of the alloy is as low as 750 to 850.degree. C. Therefore, in this case, the neodymium metal can be recovered as a molten alloy even at a temperature as low as the electrolytic temperature for producing the misch metal. In this process, the cathodic reaction proceeds in accordance with a reaction formula Nd+xFe.fwdarw.NdFe.sub.x. The recovered neodymium-iron alloy can be used as a master alloy such as a material for neodymium-iron-boron magnets.
If a nickel cathode is used in this method as the consumable electrode, a rare earth metal-nickel alloy can be obtained in accordance with a reaction formula R+xNi.fwdarw.RNi.sub.x. (R: rare earth metals)
In the above electrolytic reaction, the reaction proceeds as the charged rare earth oxides are dissolved in the fluoride molten salt bath and ionized. Thus, if the electric current is supplied at a rate higher than the dissolving rate of the oxides, the dissolved oxides are running short, which causes anode effect (wherein the anode is covered with inactive gas produced by the reaction, thereby being insulated), thereby stopping the electrolytic reaction. On the other hand, it is reported that undissolved rare earth oxides react with the fluorides in the electrolytic bath in accordance with a reaction formula Nd.sub.2 O.sub.3 +NdF.sub.3 .fwdarw.3NdOF to form oxyfluorides, which are not electrolyzed (Report of Molten Salt Committee of The Electrochemical Society of Japan, "Yoyuen oyobi Koon Kagaku (Molten Salt and Pyrochemical)" Vol. 38, No. 1, P.48 (1995)).
Accordingly, it is necessary in the electrolysis in a fluoride molten salt bath with a charge of oxides to dissolve rare earth oxides in the fluoride molten salt bath in an amount corresponding to the supply of the electrolytic current. Also inthisprocess, dissolution of rare earth oxides takes time since the rare earth oxides are first dissolved in the molten salt bath and then dissociated into ions. Thus, before completely dissolved, the rare earth oxides are sedimented at the bottom of the electrolytic cell to form a slug, thereby interrupting a prolonged electrolysis.
Improvements have been proposed, such as electrolysis of fluorides using rare earth fluorides in place of rare earth oxides (Japanese Laid-open Patent Application Nos. 61-87888 and 61-266086, and U.S. Pat. No. 496661 (1990)). Such electrolysis of fluorides, however, has some disadvantages, namely: the rare earth fluorides used in the process are more expensive than the oxides; and the gas produced by the electrolysis is a fluorine gas, which requires expensive facilities for exhaust gas treatment in order to prevent pollution.
Alternatively, in order to improve solubility of the rare earth oxides in the electrolysis with a charge of oxides, there has been proposed an electrolytic reduction method using Re.sub.2 O.sub.2 CO.sub.3 (Re stands for rare earth elements) as the starting material (Japanese Laid-open Patent Application No. 6-280077). This method has an advantage in that dissolution of rare earth oxides which are produced by decomposition of the charged Re.sub.2 O.sub.2 Co.sub.3 is promoted under the similar conditions to those for the conventional electrolysis with a charge of oxides at the bath temperature of about 1000.degree. C. However, since this method employs a high temperature bath as in the conventional method, the problems relating to shortening of life time of the electrolytic cell and electrodes are not solved. Accordingly, this method is not substantially improved in regard of this point.